Cable type electromagnetic (EM) survey systems include various electromagnetic field measurement devices, for example, spaced apart electrode pairs, magnetic field sensors, magnetometers and the like, disposed at spaced apart locations along a streamer cable. The cable may be towed behind a vessel or may be disposed on the bottom of a body of water. The measuring devices detect various components of an electromagnetic field resulting from imparting a time varying electromagnetic field into rock formations below the bottom of the body of water. The measurement devices typically have signal processing components such as amplification and communication electronics disposed proximate each such measurement device. Such electronics may include amplifiers and analog-to-digital converters or electrical-to-optical converters to communicate a representation of the measurements to a recording system on the vessel or elsewhere.
The amplification and communication devices, which are distributed over the length of the streamer cable, consume electrical power. The front-end (preamplification stage) in particular of such devices have substantial sensitivity to low frequency noise originating in the power distribution system. Methods known in the art for attenuating electromagnetic interference (EMI) originating in the power system using various types of suppressors (e.g., capacitive shunts and series connected ferrites) have little effect in the frequency range of interest for marine EM hydrocarbon exploration.
In a typical constant voltage cable power distribution system, noise may originate from current surges caused by variable load power consuming devices in the measurement system (e.g., telemetry units). The noise transfers to the amplification front-end either through the electric power lines or by inductive coupling to the electric or magnetic field sensors. One known method for suppressing this type of noise is to use a battery powered system. See, for example, U.S. Pat. No. 7,602,191 issued to Davidsson and commonly owned with the present invention. However, battery installation in the streamer cable is a drawback from a maintenance and weight point of view. It is difficult to obtain neutral buoyancy when batteries are included in the streamer. Battery powered streamers also have limited acquisition time because measurements should be avoided while charging the batteries.
There continues to be a need for improved electromagnetic streamer power systems in order to reduce noise in the recording of electromagnetic signal data.